This year was our first winter to go south. I've followed threads on this site for several years and created a load of info for both Texas and Florida. When it came down to it I decided I was partial to having more vegitation than Texas can provide. I read threads from people that wintered in Texas who found it was often windy down in the RGV and somewhat dusty. We were down there back in the 90's and also had been to Florida so we knew enough to decided we'd give Florida our first try. We liked it so much we're going back for longer next winter.
Florida parks can be more expensive than Texas. At least when comparing monthly rates for RV parks. We found some less expensive parks even further south where it is often more expensive. If you stick to the middle of the state, along US27, you will find the parks there to generally be less expensive than the coastal parks. But the beaches are certainly nice along the coasts. Still, Florida isn't so big and that allows you to visit the coastal areas even when staying in the interior.
I think you can find parks in both Texas and Florida without making reservations. We stayed in 9 different parks during out trip this past winter so we'd get a better feeling of the different regions. We only made three reservations the whole time and one of those was a state park (usually need reservations, especially in south FL). The other two reservations were in the Keys and a park near Miami, but we could have stayed in both without reservations as there were always vacancies during our stays.
Take the opportunity to go to one state or the other and travel around rather than staying in one place the whole time. Definitely do some research on this site or RVparkreviews.com. That's how we narrowed down our parks.